FIG. 1 shows a prior art implementation of a transceiving circuit for contactless communication. This transceiving circuit employs an integrated near field communication transmission module 2, e.g. type no. PN5xx, e.g. type no. PN511 or PN512 manufactured by NXP Semiconductors, and external passive electronic components. The transmission module 2 is integrally equipped with transmitter means 3 being adapted to generate an electromagnetic carrier signal, to modulate the carrier signal according to transmitting data and to drive an antenna 5 with the modulated carrier signal, and with receiver means 4 being adapted to sense response signals being received at the antenna 5 and to demodulate the response signals. The transmission module 2 has output terminals TX1, TX2 being connectable to first and second transmitting paths wherein the transmitting paths are connected to the antenna 5 which is represented in FIG. 1 by its equivalent circuit components capacitance Cext and inductance Lext. Between the output terminals TX1, TX2 of the transmission module 2 and the external antenna 5 the following components are switched into the transmitting paths: an electromagnetic compatibility (EMC) filter comprising two inductors L0 being serially switched into the transmitting paths and two capacitors C0 being connected in parallel to the antenna 5; two DC decoupling capacitors C10 being serially switched into the transmitting paths; and an impedance-matching network 6 being arranged between a receiving path tapping A and the antenna 5, wherein the impedance-matching network 6 comprises two capacitors C2a being connected in parallel to the antenna 5 and two ohmic resistors Ra being serially switched into the transmitting paths. It should be noted that the antenna 5 is “tuned” by means of trimming the impedance-matching network 6 during manufacturing of the transceiving circuits.
Further, the receiver means 4 of the transmission module 2 comprise an input terminal RX that is connected to a receiving path that branches off from the first transmitting path at tapping A. A phase adjusting capacitor C3 is switched into the receiving path in order to enable adjusting of the phase angle of signals between the first transmission path and the receiving path. By adjusting the phase angle an optimal demodulation can be achieved. Further, an ohmic resistor R1 is serially switched into the receiving path. With this resistor R1 the voltage level appearing at the input terminal RX of the receiver means 4 can be adjusted. Numeral VMID depicts an analog reference voltage input of the receiver means 4. A capacitor C4 is switched between the analog reference voltage input VMID and ground potential. An ohmic resistor R2 connects the input terminal RX and the analog reference voltage input VMID.
For a better understanding of the function of the RFID transmission module 2, a block diagram of the near field communication (NFC) transmission module type no. PN511 is shown in FIG. 2. The NFC transmission module 2 comprises analog circuitry which can be roughly divided into transmitter means 3 and receiver means 4. Although not shown, the analog circuitry comprises output drivers, an integrated demodulator, a bit decoder, a mode detector and an RF-level detector. A contactless UART communicates with the analog circuitry via a bus. The contactless UART comprises data processing means, CRC/Parity generation and checking means, frame generation and checking means, and bit coding and decoding means. The UART further communicates with a microprocessor, comprising a 80C51 core, ROM and RAM. A host interface enables to connect the transmission module to external devices. The host interface may comprise I2C, serial UART, SPI and/or USB interfaces. Further details of the transmission module can be looked up in the respective data sheets which are publicly available.
One of the most important field of application of near field communication (NFC) transmission modules are mobile phones. Mobile phones equipped with NFC transmission modules can be used for ticketing, access control systems, payment services, etc. Usually, the NFC transmission modules are powered by the hosting mobile phone.
A severe disadvantage of this known transceiving circuit is that due to the chosen manner of antenna tuning the current consumption of the near field communication (NFC) transmission module can vary in such a large range that it even drifts out of the specifications. This variation of current consumption is based on a detuning effected which occurs when an antenna 11 of an NFC card or tag (or generally speaking of a resonant circuit 10) is brought in varying proximity to the antenna 5 of the transceiving circuit as shown in the schematic arrangement of FIG. 3. In this arrangement the antenna 5 of the transceiving circuit is positioned in the center of a Cartesian coordinate system (x,y,z) and the antenna 11 of the resonant circuit 10 is moved along the z-axis. The distance n between the antenna 5 of the transceiving circuit and the antenna 11 of the resonant circuit 10 is normalized. Varying the distance in z-axis direction between the antenna 5 and the antenna 11 of the resonant circuit 10 results in varying detuning of the antenna 5 of the transceiving circuit, the consequences of which can be seen in the graph of FIG. 4. The graph of FIG. 4 depicts the current consumption I(z) of the transceiving circuit in dependence on the distance in z-axis. A horizontal line represents the specified maximal current consumption Imax. It will be appreciated that for distances between the antenna 5 and the antenna 11 smaller than the value 1 the current consumption of the transceiving circuit exceeds the specified maximal current consumption Imax and is therefore not acceptable for practical implementations. It should be mentioned that the highest coupling between the antenna 5 and the antenna 11 is reached when their mutual distance is zero. This problem of this prior art antenna matching cannot even be solved by increasing the matching impedance to considerably higher values (e.g. from values of between 35 and 60 Ohms which are usually chosen to a value of 100 Ohms). Therefore there is still a strong need for a transceiving circuit for contactless communication that allows the circuit to be operated within the current consumption limits given by the specification. Particularly, such a transceiving circuit should be able to be operated in the whole range of reading distance from zero to the maximum reading distance.